The long term goals are to define the mechanism by which HER2 regulates cyclin E expression and to prepare the applicant for a career as a physician scientist who will submit an R01 application prior to completion of the award period. The career development plan emphasizes acquisition of technical skills and expansion of relevant knowledge base to facilitate the transition to independence. The research plan investigates our novel finding that overexpression of HER2 and cyclin E is associated with an aggressive phenotype of breast cancer and that HER2 downregulation results in decreased cyclin E expression. Hypothesis: HER2 acts upstream of cyclin E in breast cancer cells regulating the generation of cyclin E and its low molecular weight (LMW) forms, which may predict response to HER2-targeted therapy or serve as a 2nd therapeutic target. Aims: 1) Identify mechanisms by which HER2 regulates LMW cyclin E 2) Establish the effect of HER2 and cyclin E overexpression on response to targeted therapy 3) Evaluate the prognostic and predictive significance of HER2 and cyclin E overexpression in breast cancer patients. Study Design: We will establish the effects of HER2 on cyclin E transcription and degradation using an isogenic MCF-7 system. The effects of HER2 on the generation of LMW isoforms by altering the elastase:elafin ratio will be determined using the 76N isogenic breast epithelial cell model system. Second, we will use a model of MCF- 7 cells stably transfected with FLAG-tagged cyclin E constructs representing cyclin E and the LMW forms and cotransfected with HER2 to investigate the effects of cyclin E on HER2- and cyclin E-targeted therapies. Results will be confirmed in vivo using an HER2-overexpressing breast cancer xenograft model. Finally, we will analyze expression of HER2, FL and LMW cyclin E in breast tissue specimens collected from patients treated at our institution. Expression levels will be correlated with clinical outcome. Relevance: Our studies will allow us to understand the mechanism by which HER2 regulates expression of cyclin E and its LMW forms. Insight into the relationship between these two known poor prognostic factors in breast cancer will determine if there is clinical utility in determining cyclin E levels to either predict response to HER2-targeted therapy or serve as a 2nd target for therapeutic agents. This knowledge will provide a strategy to determine treatment strategies using targeted therapy in patients with HER2-overexpressing breast cancer.